Body coupled communications (BCC) or body-based communication has been proposed as a promising alternative to radio frequency (RF) communication as a basis for body area networks. BCC allows exchange of information between a plurality of devices which are at or in close proximity of a body of a human or an animal. This can be achieved by capacitive or galvanic coupling of low-energy electrical fields onto the body surface. Signals are conveyed over the body instead of through the air. As such, the communication is confined to an area close to the body in contrast to RF communications, where a much larger area is covered. Therefore, communication is possible between devices situated on, connected to, or placed close to the body. Moreover, since lower frequencies can be applied than typically applied in RF-based low range communications, it opens the door to low-cost and low-power implementations of body area networks (BANs) or personal area networks (PANs). Hence, the human body is exploited as a communication channel, so that communication can take place with much lower power consumption than in standard radio systems commonly used for BANs (e.g. ZigBee or Bluetooth systems). Since BCC is usually applied in close proximity to the body, it can be used to realize new and intuitive body-device interfaces based on contact or proximity. This creates possibilities for many applications in the field of identification and security.
FIG. 1 shows a schematic diagram indicating involvement of a human body in a BCC communication system. Small-sized BCC devices without direct skin contact can be realized by exploiting capacitive coupling to the human body. A two-electrode TX device generates a variable electric field that is coupled to the human body; a two-electrode RX device senses the variable electric potential of the human body with respect to the environment. Measurements have shown that a typical body channel has a high-pass character, with a lower corner frequency determined by the input impedance of the RX device and by the capacitance of the electrodes. The signal attenuation is less than 80 dB for devices positioned at various distances on the static or moving human body. With respect to interferences, the body picks-up a significant amount of interferences in the frequency band below 1 MHz, while for higher frequencies the level of interference stays below 70 dBm and their frequency spectrum is to a great extent dependent on the environment. Hence, the established body-channel properties make the frequency band between 1-30 MHz especially attractive for BCC as this band can provide sufficient data-rate for healthcare or consumer applications (up to 10 Mb/s) and the impact of radio frequency (RF) interference is less, as the body does not act as an efficient antenna.
BCC can be technically realized by electric fields that are generated by a small body-worn tag, e.g., being integrated into a credit card or another suitable device attached to or worn in close proximity to the body. This tag capacitively or galvanicly couples a low-power signal to the body. Sometimes this body-coupled communication is referred to as “near-field intra-body communication”. BCC is a wireless technology that allows electronic devices on and near the human body to exchange digital information through capacitive or galvanic coupling via the human body itself. Information is transmitted by modulating electric fields and either capacitively or galvanicly coupling tiny currents onto the body. The body conducts the tiny signal to body mounted receivers. The environment (the air and/or earth ground) provides a return path for the transmitted signal.
FIG. 2 shows a schematic block diagram of components involved in a BCC communication system, as disclosed for example in the European Patent EP0824799B1. The wireless system comprises a transmitter TX and a receiver RX each may have two conductive plates (an inner electrode arranged close to or on the body B, and an outer electrode) used for propagating the electric field and thus form an electric circuit composed by the body B and by a return path (air and ground GND). Information I supplied to the transmitter TX is encoded in an encoder COD and then amplified in an transmission amplifier ATX. The BCC signal then propagates along a biological conductor formed by the body B to the receiver RX, where it is amplified by a receiving amplifier ARX and then decoded in a decoder DEC to obtain the transmitted information I. Thus, the transmitter TX and the receiver RX are coupled through the body B of a user and room or earth ground GND. The transmitter TX produces low-frequency, low power signals that, through capacitive coupling, pass as displacement currents into and from the body B of the user. The shared ground GND provides the return path for the current. The inner electrode may be closely coupled capacitively to the user's body B such that the “quasi-electrostatic” field resulting from the electrode potential causes a displacement current to pass to the user's body B. The outer electrode may be oriented so that its coupling to the room ground GND is stronger than that of the inner electrode, such that room ground GND acts as a return path for the current from the receiver RX. The receiver RX similarly comprises a pair of electrodes. One of the receiver electrodes is closely coupled capacitively to the user's body B such that displacement current that passes from the body B can be detected at that electrode. The signal then flows through a detector circuitry to the other electrode, which may be asymmetrically coupled capacitively to room ground GND, to complete the path for the current. The detector circuitry detects the current and operates in a conventional manner to recover the transmitted information therefrom. One or more receivers may be carried by other users or may be located in fixed positions around a room, and the return path can be a combination of air and earth ground. Accordingly, the user need not physically contact the receivers to pass information to them. Such a system may operate for example at 330 kHz and may be capable to achieve a data rate of a few kbps.
In hospitals for example, clinicians who are controlling medical imagery systems like e.g. X-Ray may be required to log-on before they use a computer and log-off afterwards. Indeed the medical data contained in the system must be very well protected and it must be ensured that no unauthorized person accesses this information. Therefore, authentication plays an important role. Current log-in procedures, for example using passwords, or fingerprints or other biometrics are very cumbersome, so that system access is slow and availability reduced.
To circumvent these time-consuming procedures, in practice, it happens often that a clinician logs-on once in the morning and logs-off once in the evening which potentially enables anyone to use the system during the day. This of course undermines the complete security of the system and is not at all acceptable.